Children who grow up with an older brother tend to become more aggressive as they move through adolescence, while those with a younger sister tend to become less so, according to a new study by researchers at UC Davis. The study appears in the latest issue of the journal Child Development.

"Siblings contribute unique opportunities for children and adolescents to practice certain types of aggressive behaviors, including teasing, threatening and fighting," said lead author Shannon Tierney Williams, who conducted the research as a postdoctoral researcher in Human and Community Development at UC Davis. "And because boys consistently demonstrate higher rates of aggression than girls, it may be that adolescents are affected more strongly by the aggression of brothers."

Williams and her colleagues conducted annual assessments of sibling pairs from 451 rural Iowa families -- 902 adolescents in all -- from 1989 through 1992. Each assessment involved a home visit, during which parents and kids completed a set of questionnaires in which siblings rated their own aggressive behaviors and parents described economic pressures on the family. Families were also videotaped, without researchers in the room, as they discussed household issues and tried to resolve a real-life family conflict. The videotapes were then analyzed for clues to family hostility, coercion, warmth and support.

The researchers found that on average, aggression increased over time in adolescents with an older brother but remained stable in those with an older sister.

For the older sibling, aggression decreased when the younger sibling was a girl but remained stable when the younger sibling was a boy.

"By having younger sisters, who typically exhibit lower levels of aggressive behaviors, particularly early in adolescence, older siblings may have fewer chances to practice their aggressive behavior," Williams said.

Parental hostility and family economic stress also played a role in adolescent aggression, the researchers found.

The findings suggest that efforts to address adolescent aggression should involve siblings and families, Williams said.

Her co-investigators were Katherine Jewsbury Conger, an assistant professor of human and community development at UC Davis, and Shelley Blozis, an assistant professor of psychology.